It's OK, it's so much simpler than I thought
In config.php, just get rid of all occurrences of "
http://www.yoursite.com". You just don't need those things. Now you can browse by HTTP or HTTPS as you see fit.
Only remaining issues for HTTPS Admin are:
1. "View Site" link (from Admin page) fetches some pages by HTTP. This does means that your Session cookie goes out in clear-text, but intercepting this isn't enough for an eavesdropper to gain Admin rights...
2. When you log in successfully to the Admin page, there are some more cookies set:
Set-Cookie: cms_admin_user_id=1
Set-Cookie: cms_passhash=123xxxxxxxxxxxxxxxxxx456
Really, these Cookies ought to be marked Secure if we come in over HTTPS, and they should probably be given "Path: /admin" so that buggy browsers don't send them when they're not needed. But it all seems to work properly anyway (at least in Firefox).
It might be slighly more secure if the admin userID and password hash was not sent on every admin request. As a possible alternative you might consider a second session cookie (marked Secure) for those GETs and POSTs that require authentication. It might also be good to allocate user ID's randomly, just to make brute-force attacks harder.
Sorry if some of this seems pedantic. My day job is in security
